Reactive sputtering systems are finding increasing use for the manufacture of multi-layer thin film coatings. Multi-layer thin film coatings comprising alternating layers of two or more materials are typically formed in sputtering systems by alternate operation of two (or more) targets composed of the base materials. In a reactive sputtering system, a reactive gas is introduced into the sputtering chamber so that atoms of the material sputtered from the target onto the substrate(s) are converted into the desired compound such as a metal oxide or metal nitride. In reactive sputtering systems, it is desirable to operate the system so that all of the sputtered material is converted to the desired chemical compound without poisoning the sputtering target. However, it is difficult to maintain the target in the desired non-poisoned mode while still obtaining stochiometric oxidation of the sputtered material in the growing film. If an insufficient amount of reactive gas (e.g. oxygen) is present, the growing film will be absorbing and have incorrect stochiometry. If the amount of reactive gas is too large, the target becomes poisoned, i.e., the sputtering surface of the target reacts with the reactive gas to form a compound (e.g. an oxide). Once the target becomes poisoned, the deposition rate greatly decreases compared to an unpoisoned target.
It is critical to maintain a stable reactive gas concentration in the reactive sputtering chamber at the desired level for optimum performance of the sputtering system. Thus the reactive gas concentration provides one critical process variable. The use of a steady state control to admit the reactive gas into the sputtering chamber would not be an adequate control mechanism because the consumption rate of the reactive gas varies with the state of the target and the conditions inside the sputtering chamber.
It is well known to employ a control algorithm, such as a PID (Proportional, Integrative, Derivative) loop, to control the admission of reactive gas into the chamber to maintain steady state operation of the target. Control of the flow rate of reactive gas into a reactive sputtering chamber is important to ensure stochiometry of the thin film throughout a layer while maintaining the relatively high sputter rates of the target in an un-poisoned state.
The difficulty of maintaining steady state operation of the target in the desired mode are greatly increased during the first few seconds of operation after target start-up. In prior art reactive sputtering systems having a well-tuned PID loop to control the reactive gas concentration in the chamber, it typically takes between four to eight seconds to stabilize the reactive gas concentration at the desired steady-state level. These stabilization times are about the best that can be achieved due to the nature of the PID loop and computing hardware that is typically employed in prior art systems.
In the manufacture of multi-layer thin film coatings where the process of forming the coating requires repetitive target starts, the time required to achieve steady-state operation of a target can be crucial for accurate control of stochiometry, deposition rates, and hence the optical performance of the coating. Undesirably long stabilization times can result in poor optical performance of the completed film. In the ideal case, the target would achieve steady state, controlled operation sufficiently quickly that the substrates being coated are never subjected to film deposition during process stabilization.
The problem of film inhomogeneity during target start-up and stabilization are particularly acute in a drum configuration sputtering system where the drum carriage typically rotates at about 45 to 120 rpm. At these rotation rates, each substrate passes under the target between four and eight times during target stabilization, leading to significant deposition of inhomogeneous layers in the coating. This is particularly true of thinner layers where the time needed for target start-up and stabilization forms a significant percentage of the overall layer deposition time. For a drum configuration reactive sputtering system, it would be of significant advantage to have the target brought to stable, steady-state operation and to have the reactive gas flow stabilized in less time than required for one revolution of the drum, i.e., in less than one second.
Two general methods have been employed in the prior art to solve the problem of poor process control during target start-up. PID control of the oxygen flow during the main body of the layer deposition can be used, but during layer start-up the stabilization time of a PID loop is too long to provide good performance. To address the poor performance of a PID loop during target start-up, various approaches are used. One method includes ramping the power to the sputtering target on a pre-determined profile to reach the steady state value. The reactive gas flow may be ramped in accordance or set to a fixed value during stabilization, with reactive gas flow control being returned to the PID loop after target stabilization. However, neither of these approaches achieve steady-state operation of the target in a desirable time frame. This causes the film composition to be inhomogeneous over a complete layer. For multi-layer designs incorporating various layer thicknesses the contribution to the total thickness of any given layer can vary markedly, making the attainment of precision optical designs problematic.
Another method employed in the prior art includes the use of target shutters that shield the target during target start-up to prevent the deposition of target material on the substrates until the target stabilizes. Once the steady state condition has been achieved the target shields are removed and deposition on the substrates begins. The difficulty with this approach is that as soon as the shutters are removed, the conditions at the target change rapidly (influx of oxygen from areas beyond the shutters and so forth) requiring the system to reach a new steady-state operating condition. During this transition time, the substrates (or some portion of a large substrate) may be exposed to varying fluxes of sputtered material resulting in non-uniform deposition over the substrate or substrates. The shields also require undesirable extra mechanical hardware that must be manipulated in the vacuum environment, adding expense and requiring additional maintenance and downtime of the system.
The target start-up and time frame needed to reach a stable operating process is not the only area in which a standard PID loop provides insufficient control to maintain a coating system in a stable state. Metal oxides are commonly used as coating materials, with the stability of the system during coating being determined by various factors such as oxygen flow, pumping of the chamber, and the rate of consumption of the oxygen in the oxidizing process. For any given system (with no targets running and no film being deposited), a pumping curve showing pressure vs. reactive gas flow can be determined experimentally, the pressure for a given flow being dependent on variables such as the speed of the vacuum pump and the volume and configuration of the sputtering chamber. When the system is running and depositing material on substrates, the pressure for a given reactive gas flow changes because the film being deposited also functions as a “pump” for the reactive gas by consuming the free oxygen into the growing thin film.
The magnitude of the “pumping” effect the film has on the reactive gas pressure depends on several variables specific to each machine configuration and operating parameters. The behavior of a system at various flows of reactive gas follows a well known characteristic curve.
A set of such curves are shown in FIG. 1. With reference to FIG. 1, the pump line 10 shows the behavior of the system when no target is operating, while the other three curves 20, 30, and 40 show the system states for three different powers at which a target operates. The curve 20 shows that at a low power, when a relatively small amount of material is being deposited, there is little deviation from the standard pump curve. As the power to the target is increased, the pumping effect of the film becomes greater, leading to the characteristic S-shape seen in the curves 30 and 40. The regions of these S-shape curves having negative slopes correspond to non-equilibrium states of the system and, in practice, control is not sufficient to allow one to follow these curves as the reactive gas flow increases, especially for large industrial systems. Instead the target will poison, causing the system to suddenly jump from one point on the curve to another, as illustrated by the dashed arrow 45 on the curve 40. As the pressure decreases, the system will then jump back down to a metallic mode as illustrated by the arrow 47 resulting in a hysteresis curve. The behavior of reactive sputtering systems on these operating curves is well known.
It is also known that operating states giving the best film qualities often require one to operate very near conditions in which the system is no longer stable. Two such states are those that occur just below and just above the knee 42 on the curve 40. If the system is operating at a point along the curve 40 that is too far to the left of the knee 42, then the target is operating in the metallic mode which, although providing a rapid coating rate, often results in formation of absorbing films with materials such as Nb2O5 and TiO2 that are typically difficult to oxidize.
The closer the system is to the knee of the curve, the more oxidized the deposited film becomes, but the greater the likelihood that any perturbation in system conditions will cause the system to become unstable before a standard PID control system can compensate for the changed conditions. It is also possible to operate just above the knee of the curve where the target operates in a barely poisoned condition. This operating point also suffers from the difficulties in keeping the system stable and preventing any small change in conditions from moving the system into an unstable state.
There remains a need for improved control in reactive sputtering system. It is an object of the present invention to obviate the deficiencies of the prior art reactive sputtering systems. This and many other of the objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.